A conventional electromagnetic spring clutch is shown in FIG. 4.
A field core 400 is made of magnetizable material and formed in a bottomed cylinder. There is bored a throughhole 406 through which an axle 404 is inserted in the bottom section 402 of the field core 400.
An electromagnetic coil 408 is fixed in the field core 400 and fixed therein by suitable means such as caulking, adhering, etc.
An armature 410 is also made of magnetizable material and there is provided a flange 412 on the outer face thereof.
When the electric current passes the electromagnetic coil 408, then the armature 410 is attached to shift toward the electromagnetic coil 408. With the shift of the armature 410 in the axial direction thereof, the inner diameter of a coil spring, not shown, is reduced or increased to control whether torque is transmitted or not transmitted. Note that, the magnetic circuit is closed via the field core 400, the flange 412 of the armature 410 and the cylinder section of the armature 410 when the electric current passes through the electromagnetic coil 408.